488 
PROFESSOK HENRY A. MIERS; AN ENQUIRY INTO 
III the case of a lirig-ht octahedron of alum, the first effect of slight dilution or 
warming is to develop small pits having the form of equilateral triangles inscribed in 
the triangle which bounds the octahedron face; the triangles are small shallow pits 
and their sides must therefore be vicinal faces, which are still triakis-octahedra, like 
those of the growing crystal. 
Sometimes the growing crystal has its octahedron face covered with fiat triangular 
plates whose outline is parallel to that of the octahedron face ; when the solution is 
diluted, these have their corners rounded and become converted into hexagonal plates, 
then disappeai' and become replaced by the inscribed triangular ]3its. 
In one case the prerosion faces (belonging to a triakis octahedron) on one ■ 
octahedron face made an angle of 0° 57' with each other; the crystal was then 
allowed to grow during the night, and in the morning well defined vicinal planes had 
made their appearance, making an angle of 0° 15' with each other ; as the 
temperature rose these rvere replaced by prerosion faces belonging to different forms; 
that on one edge being inclined at about 0° 5' to the true octahedron, and that on the 
adjacent edge at about 0° 18'. This feature was observed in other etching 
experiments ; the prerosion faces replacing an octahedron face generally belong to 
different forms, whereas the vicinal jilanes of growth generally belong to the same foim. 
In one case two well defined prerosion faces belonging to the form [221 j made 
their appearance upon an octahedron edge, inclined at 15 46 and 15 41 respecti’v el} , 
to the adjacent octahedron faces ; the theoretical angle for 
[221] is 15° 47' 36". 
The prerosion faces 8 which replace the edges of the vicinal 
triakis octahedron a, /3, y of fig. 15 belong to a different form, 
the icositetrahedron ; and, since the first eftect of etching is 
to round off* the edges of the crystal, it generally results in the 
production of such icositetrahedra ; but they do not necessarily 
truncate the edges a &c. Thus the crystal just mentioned 
yielded the following results three days later, when the- 
corroding action of the solvent happened to be extremely slow. 
Nocemher 'll, 1893.—At LO a.m., each octahedron face yielded three very perfect 
images ; the actual readings are given on p. 473. A little later, the form replacing o 
was measured and was found to be a triakis-octahedron inclined at 4^ to o (pieMOUsl} 
determined as 5'). At 10.45 the edges of this vicinal form were replaced by facets of 
an icositetrahedron 8 inclined at almost exactly the same angle to o , and the same 
tiling happened on the other laces of the crystal. Ihe angle between 8 and the tiue 
octahedron face was measured as 0° 2' on o ; 0° 4' on w'; 0 3T' on o. The triangulai 
etched pits did not begin to make their appearance until 3.15 r.M. 
On the following day the same events recurred : the vicinal laces of giowth 
replacing o belonged to a triakis-octahedron Inclined to o at 0 2^, and at 11.15 a.m. 
the edges of this form 
wmre replaced by prerosion faces belonging 
to an 
